Various methods have been proposed for the separation of components included in liquid, gas, and other fluids. For example, spiral separation membrane elements are widely used for the removal of ionic substance in sea water, brackish water, and the like. Exemplary separation membranes used in a spiral separation membrane element include microfiltration membrane, ultrafiltration membrane, nanofiltration membrane, reverse osmosis membrane, and forward osmosis membrane. These separation membranes are used, for example, in the production of drinking water from sea water, brackish water, water containing toxic substances, and the like, as well as in the production of commercial-scale ultra-pure water, wastewater treatment, and recovery of valuables. The separation membranes used are selected depending on the target component to be separated as well as the separation performance.
Spiral separation membrane elements are used by supplying raw fluid to one surface of a separation membrane and obtaining permeate fluid from the other surface. One merit of the spiral separation membrane elements is the large separation membrane area compared with other separation membrane elements.
A spiral separation membrane element comprises a wound body formed by spirally wound separation membranes, an upstream end plate fitted on one end of the wound body, a downstream end plate fitted on the other end of the wound body, a raw fluid channel along one surface of the separation membrane and a permeate fluid channel on the other surface of the separation membrane, and a permeate fluid collection tube, wherein the wound body is formed by spirally winding the separation membranes around the permeate fluid collection tube with the raw fluid channel being closed to the permeate fluid collection tube and the permeate fluid channel being open to the permeate fluid collection tube, one end of the permeate fluid collection tube is closed and the other open end is located in the exterior of the downstream end plate, raw fluid is supplied into the raw fluid channel though the upstream end plate, concentrate fluid which failed to permeate through the separation membrane is discharged through the downstream end plate, and permeate fluid which has permeated through the separation membrane is discharged through the permeate fluid collection tube.
When the raw fluid is water, and the water is treated by using a spiral separation membrane element prepared by using a reverse osmosis membrane for the separation membrane, a polymer net is often used as a channel material for the formation of a channel, and provided on the feed water side of the separation membrane. The separation membrane commonly used is a compound semipermeable membrane comprising a separation functional layer of a crosslinkable high molecular weight compound such as polyamide, a porous support membrane of a high molecular weight compound such as polysulfone, and a nonwoven fabric of a high molecular weight compound such as polyethylene terephthalate disposed in this order from the feed water side to the permeate water side. A channel material is also used on the permeate water side to facilitate the flow of the permeate water along the separation membrane, and also, to prevent falling of the separation membrane into the channel and secure the flow path of the permeate water. The material often used for the channel formation is a fabric such as tricot which has a smaller channel interval smaller than the channel material on the feed water side.
For the water production apparatus using the spiral separation membrane element, improvement of the water production performance has been demanded to thereby reduce the cost of water production. To improve separation performance of the spiral separation membrane element, there have been various proposals to increase the amount of the permeate fluid produced per unit time by various improvements in the performance of the separation membrane layer which has channel members for respectively defining feed water channel and permeate water channel in the element and which is spirally wound.
For example, JP 2006-247453 A proposes a method wherein a sheet member provided with projections and depressions is used for the channel material on the permeate water side. JP 11-114381 A proposes use of a separation membrane having projections and depressions formed on the feed water side of the separation membrane and a hollow channel formed in its interior, without using any substrate (channel material) for the channel formation. JP 2010-099590 A proposes use of a sheet-form compound semipermeable membrane comprising a porous support layer having projections and depressions formed thereon and a layer having separation activity without using the channel material such as net on the feed water side or the channel material such as tricot on the permeate water side.
In the meantime, use of a plurality of separation membranes in the spiral separation membrane element with the separation membranes folded such that the surface on the feed water side opposes with the surface on the feed water side of adjacent separation membrane has been known in the art. This use of folding has enabled provision of the channel material such as a net at a relatively high precision since the channel material is sandwiched between the surfaces of the separation membranes on the feed water side. The folded separation membrane pairs are disposed one on another with the surface of on the permeate water side opposing the surface of on the permeate water side of the adjacent folded separation membrane pair, and then used for the wound body.
Each separation membrane pair has a crease (fold) on one side, and the raw fluid channel in the interior of the separation membrane pair is closed by this crease in relation to the permeate fluid collection tube. One of the sides in the direction perpendicular to the direction of the crease (one of the sides extending in the axial direction) opposes the upstream end plate and the other side of the sides in the direction perpendicular to the direction of the crease (the other one of the sides extending in the axial direction) opposes the downstream end plate to constitute the wound body. The remaining one side of the separation membrane pair is closed by adhesion.
The conventional spiral separation membrane elements as described above are not sufficient in improving their performance, particularly in improving the stability of the separation performance in the long term operation. The method proposed in JP 2006-247453 A using a sheet member having projections and depressions formed on the permeate water side for the channel material only reduces the flow resistance of the permeate water. In addition, the space allowed for the fluid flow in that method is smaller than the case of the separation membrane having the projections and depressions formed directly thereon because of the thickness occupied by sheet material itself, and therefore, the effect of reducing the flow resistance of the permeate water has been insufficient.
In the method proposed in JP 11-114381 A wherein the separation membrane has projections and depressions formed on the side of the feed water and a hollow channel formed in its interior without using the substrate, the separation membrane has the hollow channel extending in the direction parallel to the surface of the separation membrane in its interior. Accordingly, the height difference between the projections and the depressions on the surface of the separation membrane can not be increased beyond certain limit, and the shape of the projections and the depressions is also limited. Furthermore, the channel in the Examples of the JP 11-114381 A is a groove having a step height of 0.15 mm. The shape of the channel on the permeate water side is also limited, and the effect of reducing the flow resistance is insufficient in both the channel on the side of the feed water and the channel on the permeate water side.
For the method proposed in JP 2010-099590 A using a sheet-form compound semipermeable membrane comprising a porous support layer having projections and depressions formed thereon and a layer having separation activity without using the channel material such as a net on the side of feed water or the channel material such as a tricot on the side of feed water, JP 2010-099590 A does not disclose the performance when the spiral separation membrane element is actually prepared by using this sheet-form compound semipermeable membrane except for the membrane performance evaluated by using a cell for evaluating the flat membrane. When the spiral separation membrane element is operated with the pressure actually applied, both the channel on the side of the feed water and the channel on the side of the permeate water are likely to experience change in their cross-sectional area, and when this element is operated not only for a short period but for a long period, the element is likely to experience change in its performance.
In addition, when a separation membrane pair is prepared by folding the separation membrane as in the case of the prior art production, folding of the separation membrane may be insufficient, and in such a case, some space or gap is left near the crease. The resulting membrane element may suffer from fluid leakage when the spiral separation membrane element is prepared by winding such separation membrane pair around the permeation fluid collection tube. In such case, the spiral separation membrane element cannot fulfill its function.
It could therefore be helpful to provide a spiral separation membrane element which does not experience sliding between the membranes in the production of the pair of separation membranes, and which stably realizes good separation function for a long time.
We thus provide a spiral separation membrane element comprising:
(a-1) a wound body comprising spirally wound separation membrane,
(a-2) a raw fluid channel provided along one surface of the separation membrane,
(a-3) a permeate fluid channel provided along the other surface of the separation membrane, and
(a-4) a permeate fluid collection tube; wherein
(a-5) the raw fluid channel is closed to the permeate fluid collection tube, and the permeate fluid channel is open to the permeate fluid collection tube,
(a-6) the separation membrane is wound around the permeate fluid collection tube to constitute the wound body,
(a-7) raw fluid is supplied to the raw fluid channel from one end of the wound body,
(a-8) concentrate fluid which did not permeate through the separation membrane is discharged from the other end of the wound body, and
(a-9) permeate fluid which has permeated through the separation membrane is discharged from the permeate fluid collection tube, wherein
(b-1) the spiral separation membrane element has at least two separation membrane pairs, and a surface in contact with the raw fluid of one separation membranes opposes a surface in contact with the raw fluid of the adjacent separation membrane to form the raw fluid channel, and the raw fluid channel between the edge portions on the side of the permeate fluid collection tube is closed by a sealing material provided on the edge portions of the separation membranes, and
(b-2) the wound body is formed by spirally winding each separation membrane pair around the permeate fluid collection tube.
It is preferable that the sealing material has a width in the direction perpendicular to the axial direction of the permeate fluid collection tube of 5 mm to 100 mm.
It is preferable that the sealing material has a thickness of 5 μm to 500 μm.
It is preferable that the raw fluid channel is formed by projections and depressions formed on the surface of the separation membrane or a channel material provided along the surface of the separation membrane.
It is preferable that a difference in height between the projections and the depressions formed on the surface of the separation membrane or thickness of the channel material provided along the surface of the separation membrane is 80 μm to 1000 μm.
We also provide a method of producing a spiral separation membrane element, comprising the steps of:
(a) preparing at least two pairs of separation membranes, wherein each pair of separation membrane is prepared by arranging two separation membranes so that a surface of one separation membrane in contact with raw fluid and the surface of the other separation membrane in contact with the raw fluid oppose each other to thereby define a raw fluid channel between the two separation membranes, bonding edge portion on one side of the separation membrane with edge portion on one side of the other separation membrane by using a sealing material so that the raw fluid channel is closed by the sealing material to thereby prepare a pair of separation membranes;
(b) producing a laminate of the pairs of the separation membranes by disposing the at least two pairs of the separation membranes prepared in the step of preparing the separation membrane pairs one on another so that the surface in contact with permeate fluid of one separation membrane pair opposes with the surface in contact with the permeate fluid of the adjacent separation membrane pair to thereby define a permeate fluid channel between the opposing separation membrane pairs with the side of the permeate fluid channel open at the side as described above for the separation membrane; and
(c) producing a wound body by winding the laminate of the separation membrane pairs prepared in the step of preparing the laminate around a permeate fluid collection tube having holes for collecting the permeate fluid in its peripheral surface so that the open section of the permeate fluid channel corresponds to the collection holes of the permeate fluid collection hole.
In our spiral separation membrane elements, sliding of the membranes between the opposing separation membranes is suppressed and, therefore, separation performance of the spiral separation membrane element of the invention is stably retained for a long time.